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Thiruvallur district, located in the northeastern part of Tamil Nadu has a total population of 22,11,413 as per 1991 Census, about half of which resides in rural areas. The district has been subdivided into eight taluks and 14 blocks for administrative convenience. Agriculture is the main occupation of the rural populace in the district and the net sown area accounts for about 40% of the total geographical area of the district. Paddy, Groundnut, Sugarcane, pulses and Banana are the important crops grown in the district. Dug wells are the major sources of irrigation, followed by tanks and bore/tube wells. The district receives an average annual precipitation in the range of 950 to 1150 mm., with the coefficient of variation ranging from 23 to 27%. Rains are received during both southwest (June to September) and northeast (October to December) monsoon seasons. There is considerable spatial variability in the distribution of rainfall, which shows a gradual decrease from east to west. Thiruvallur district falls under Araniyar – Korattalaiyar and Cooum river basins. All the rivers are seasonal and carry substantial flows during the monsoons only. The prominent geomorphic units identified in the district from Satellite Imagery include alluvial plains, old river courses, coastal plains, shallow and deep buried pediments, pediments and structural hills. The district has considerable quantum of surface water resources in the form of reservoirs and tanks and anaicuts across major streams. Poondi reservoir in Thiruvallur taluk, Red Hills tank in Ambattur taluk and Sholavaram tank in Ponneri taluk are the important reservoirs in the district, whereas Kesavaram Anaicut, Thamaraipakkam Anaicut, Vallur Anaicut are the major surface water storage structures constructed across rivers flowing through the district. Geologically, the district is underlain by formations ranging in age from Archaean to Recent. Crystalline rocks comprising Charnockites, gneisses and associated rocks are restricted to the western part of the district. The central and eastern parts are underlain by a thick pile of Gondwana shales, clays and sandstones below the Recent alluvial deposits. Geophysical investigations carried out in the district have established the presence of multi-layered substratum, indicating fracture zones in the crystalline formations and multiple granular zones in porous formations. A number of exploratory bore/tube wells have been constructed in the district under different programmes of Central Ground Water Board, which has brought out the wide spatial variation in well yields and aquifer parameters in both crystalline and porous formations. It has been established that the shallow alluvial aquifers flanking the major stream courses have prolific yields whereas the Gondwana sediments form aquifers of poor to moderate potential with considerable variation in the quality of formation water. The important aquifer systems in the district are constituted by (i) unconsolidated and semi-consolidated formations and (ii) weathered and fractured crystalline rocks. Ground water occurs under phreatic conditions in the shallow zones in Recent alluvial deposits, Gondwana sediments and weathered residuum in crystalline formations. It is under semi-confined to confined conditions in deeper sedimentary aquifers and fracture zones in hard rocks. Depth to water levels in observation wells of CGWB tapping the phreatic zone ranged from 2.44 to 9.20 m.bgl during May 2000 and from 1.48 to 9.65 m.bgl during January 2001. The shallowest water levels during pre-monsoon period were observed in Poonamalle, Villivakkam, Puzhal, Pallipattu, R.K.Pet and Thiruttani blocks whereas, during post-monsoon period, water levels were the shallowest in Pallipattu, R.K.Pet and Thiruttani blocks. The elevation of water table in phreatic aquifers during January 2001 ranged from 5.38 m below Mean Sea Level at Athipattu to 86.70 m. above Mean Sea Level at Thiruttani. The gradient of ground water table, in general, is towards the Bay of Bengal. A ground water trough with water table below Mean Sea Level is observed close to the coast in parts of Minjur, Sholavaram and Puzhal blocks. The fluctuation of water levels during January 2001 in comparison with water levels during May 2000 ranged from a decline of 3.37 m. to a rise of 2.09 m. Decline of water levels were observed in about 53% of wells analysed, indicating recharge insufficient to compensate withdrawal in more than half of the district. Decline in water levels is observed predominantly in Gummidipoondi, Uthukottai, Minjur, Sholavaram, Thiruvallur, Kadambattur and Puzhal blocks. Rise in water levels observed in about 47% of wells analysed is between 0.0 and 2.0 in a major part of the district. Rise in water levels is observed predominantly in Pallipattu, R. K. Pet, Thiruattani, Thiruvelangadu, Poondi and Villivakkam blocks. Comparison of water levels during May 2000 with the average water levels for the period 1990–99 indicates a decline in 63% wells analysed. However, analysis of long-term trend of ground water levels during May for the period 1991–2000 has shown a rising trend in about 57% of wells analysed. The depth to piezometric surface in the district ranged from 5.50 to 23.90 m.bgl during May 2000 and from 4.00 to 24.00 m.bgl during January 2001. It was below 10.00 m.bgl in a major part of the area during both these periods. Elevation of piezometric surface during January 2001 ranged from 14.00 m. below Mean Sea Level to 170.00 m. above Mean Sea Level. A ground water trough is observed near the coast in Minjur and Sholavaram blocks. The gradient is toward the Bay of Bengal in general. The flow pattern of the piezometric surface is more or less similar to that of the water table. The fluctuation in piezometric levels between May 2000 and January 2001 ranged from a decline of 1.40 m. to a rise of 11.50m. Computation of the ground water resources available in the district based on 1984 methodology show that the total replenishable ground water resources available in the district is of the order of 89410 ha.m/yr. The net utilizable ground water resources available in the district for irrigation have been computed as 68405 ha.m/yr. The net ground water draft through various abstraction structures is of the order of 65116 ha.m/yr. The balance ground water available, computed as the difference between ground water resources available for irrigation and net ground water draft has been computed as 21457 ha.m. The level of ground water development projected to year 5 and assuming an annual increase of 0.6% in ground water draft ranges from 17.33% in Gummidipoondi block to 181.70% in Minjur block. Based on the level of ground water development, Minjur, Puzhal, Poondi, Thiruvallur, Thiruvelangadu and R. K. Pet blocks have been categorised as ‘OVEREXPLOITED’ ( >100 %), whereas Poonamallee and Sholavaram blocks have been categorised as ‘DARK’ (85–100%). In Thiruttani and Villivakkam blocks, the level of ground water development is between 65 and 85% and is hence categorised as ‘GREY’. The remaining blocks, where the level of ground water development is below 65%, have been categorised as ‘WHITE’. The status of irrigation potential from ground water varies considerably from block to block. Considerable potential is available for development in Gummidipoondi, Kadambattur and Ellapuram blocks of the district, to be developed through suitable ground water abstraction structures. The balance irrigation potential available in the district can be fully developed by constructing 25544 ground water abstraction structures, each having an average command of 1.2 ha. The ground water in the phreatic and deeper zones, in general, is potable. Considerable spatial variation exists in the chemistry of ground water in these aquifers. Majority of samples collected from phreatic zone may cause high to very high salinity hazard and medium to high alkali hazard when used for irrigation. Proper soil management strategies are to be adopted in these areas while using these waters for irrigation. Majority of samples collected form the phreatic zone belong to end-product waters rich in Sodium and Chloride ions. Other geochemical processes such as cation exchange, Base Exchange etc. are also active in this zone. Major ground water–related issues in the district include the over-exploitation of ground water from the shallow alluvial aquifers in Minjur area for water supply to Chennai city, resulting in sea water intrusion into these aquifers to a distance of about 15 km. inland and pollution of ground water resources in Manali–Amabttur area by industrial effluents. In view of the rather high level of ground water development in many of the blocks of the district, further development of ground water has to be carried out in a judicious manner. Artificial recharge through rainwater harvesting also needs to be popularised in the district as a cost-effective method for arresting the decline of ground water levels and water quality deterioration. |